Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member; an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image bearing member after the toner image is transferred from the image bearing member onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
Typically, an image forming apparatus includes an image bearing member and a counter member disposed opposite the image bearing member. The image bearing member and the counter member form a transfer nip therebetween, at which an image can be transferred from the image bearing member to a recording medium such as a sheet of paper, etc. The counter member is pressed toward the image bearing member by a pressing device to contact the image bearing member to form the transfer nip. The counter member can be separated from the image bearing member using a moving device.
When a recording medium is relatively thick, shock jitter may occur at the transfer nip, and an undesirable imaging problem such as banding (i.e., uneven image concentration appearing as lines on an image) may occur. Such banding occurs when the thick recording medium enters the transfer nip, because the image bearing member receives a greater load abruptly, causing the linear velocity of the image bearing member to drop sharply.
To address such difficulty, in one approach, a rotatable cam is used to separate forcibly the counter member from the image bearing member. In this approach, a transfer roller is used as the counter member. The transfer roller includes a cylindrical roller body and a shaft projecting from both end of the roller body. The roller body and the shaft rotate integrally. Further, the rotatable cam is disposed at each end of the shaft and can rotate idly at each end of the shaft.
The rotatable cam, which can rotate idly about an outer surface of the shaft, has a convex portion at a given rotation angle position that contacts an axial end portion of the image bearing member such as a photoconductor. As the convex portion of the cam comes into contact with the transfer roller being pressed toward the photoconductor by a pressing device, the transfer roller can be separated forcibly from the photoconductor against the force so that a shaft-to-shaft distance between the photoconductor and transfer roller can be adjusted. For example, when thick paper is used as the recording medium, the transfer roller can be forcibly moved away from the photoconductor by the rotatable cam so that a transfer pressure is reduced.
With such a configuration, a sharp load increase at the photoconductor, which occurs when thick paper enters the transfer nip, can be suppressed or prevented. However, although the sharp load increase at the photoconductor can be prevented or suppressed by increasing the shaft-to-shaft distance, as a drawback, the transfer pressure is reduced, causing a transfer failure.
In another approach to prevent shock jitter, a rotatable cam rotates to separate the transfer roller from the photoconductor so as to form a minute gap therebetween before thick paper as a recording medium enters the transfer nip, thereby suppressing or preventing shock jitter. Immediately after the leading edge of the thick paper enters the minute gap, activation of a solenoid is canceled to cancel forced separation of the transfer roller so that the transfer roller can be pressed toward the photoconductor by a force of a spring used as a pressing device.
With such a configuration, the transfer roller is separated from the photoconductor until a recording medium such as a thick sheet of paper enters the transfer nip, but a sufficient transfer pressure is secured even after the forced separation of the transfer roller is cancelled.
Although advantageous, when separation of the transfer roller is canceled, the image bearing member, the recording medium, and the transfer roller may instantly collide with each other due to the force of the spring (pressing device), thereby causing a load increase or vibration at the image bearing member with possible image failure (or image deterioration) as a result.